An array-type magnetic disk controller has been in use as an I/O controller for supplying preventive maintenance information about a spare I/O unit, to be used in place of an I/O unit where an abnormality is found, to a host unit. An array-type magnetic disk controller comprises a host unit interface controller, a subordinate unit interface controller, a processor, and various registers. A plurality of data disk drives, one parity disk drive, and one spare disk drive are connected as subordinate units and are under synchronous rotation control.
An array-type magnetic disk controller checks for data abnormality by reading data and parity, parallel from each of a plurality of data disk drives and the parity disk drive. When no abnormality is found and a normal operation is confirmed, data is transferred to a host unit unmodified.
When any data abnormality is detected, an array-type magnetic disk drive restores correct data on the basis of data and parity which are read from a normal data disk drive, other than the one disk drive the defective data is read from, and a parity disk drive, and transfer it to the host unit.
If, for some reason, any abnormality including a read error or a hardware failure occurs more frequently than the specified times in one of the data disk drives acting as a current I/O unit, or in a parity disk drive, all the data written in the abnormal data disk drive or the abnormal parity disk drive is automatically and instantly recovered from the other normal data disk drives and a normal parity disk drive and restored in the spare disk drive. By using the spare disk drive as a current I/O unit, in place of the abnormal disk drive, continued operation is ensured and system down time is prevented.
However, it could happen, that the presence of a hard error or a read error in a spare disk drive itself cannot be readily detected by a host unit, and the spare disk drive itself is found to contain a hard error or a read error the first time the spare disk drive is used as an I/O unit in place of the abnormal disk drive, and cannot perform the function of a spare disk drive.
That is why a method has been in use, in an equipment comprising a main system consisting of a plurality of units, and a spare unit for backing up the units of the main system, wherein a spare unit is monitored by providing a spare unit with the same input signal as the one given to any unit of the main system, and comparing both signals output from the spare unit and the aforementioned unit of the main system (Japanese Laid-Open Patent Application 56-72359). However, this method of monitoring necessitates always operating the spare unit concurrently with the current unit in the main system. It is equivalent to having one more I/O unit to be controlled by an I/O controller, and results in increased load on the host unit.
Another method has been in use wherein abnormality of an I/O controller is regularly monitored according to a time table (Japanese Laid-Open Patent Application 62-212856). This does not, however, either monitor abnormality of a spare I/O unit or utilize a time interval not in use for an operation. Still other methods have been in use, including a method of monitoring, wherein an equipment, provided with dual central processing units (CPU), namely the current and spare CPUs, allows a test to be carried out under a condition similar to a normal operation by running a software in the spare CPU concurrently (Japanese Laid-Open Patent Application 2-93953), and a method of monitoring wherein a test is carried out at a regular interval on the spare CPU (Japanese Laid-Open Patent Application 62-90068). However, these methods of equipment monitoring having dual CPUs are not the ones for monitoring a spare CPU utilizing a time interval not in use for an operation. The present invention was developed in view of these considerations.